1. Field of the Invention
This invention relates to methods for forming flat pattern equivalents for curved surfaces. It is particularly useful in accurately transforming originally flat deformable materials such as sheet metal, glass and woven cloth into precise compound curved surfaces.
2. Description of the Prior Art
The method of this invention is particularly useful but not limited to the fabrication of high strength resin-matrix composites which are formed into parts with complex curved shapes. The term "composite" refers to material made up of two or more separate components. Carbon fiber resin matrix composites are particularly useful because of their high strength-to-weight ratio and high resistance to crack growth. One method of making a carbon fiber resin matrix composite component is to impregnate the carbon fiber fabric with resin, partially cure the impregnated fabric to a "B" stage, bring the separate layers or plies of the fabric together in a suitable tool/mold, and then completely cure the laminate under suitable heat and pressure conditions.
On a flat, constant thickness component, cutting the laminate is a simple operation. However, for components with contours or compound curves, a flat pattern must be made for each ply or sets of plies. There are a number of methods to approximate the flat pattern equivalent for the compound curve shape; one of the most common is known as the "triangulated flat pattern" method. In this method the compound curve shape is first approximated as a ruled surface, and then triangulation is used to transform the points from a three dimensional to a two dimensional surface. Unfortunately, as the complexity of the compound surface increases, the fit of the flat pattern becomes increasingly dependent upon the skill, judgment and experience of the designer and the inaccuracy of the assumption of a ruled surface. When dealing with complex shapes, unless the skill level of the designer is quite high, commonly the flat pattern will not fit. Since a composite part is often made up of as many as 150 layers of fabric, the task of obtaining flat patterns is laborous and susceptible to errors. Indeed, some complex shapes are so intricate that they may not be flat patterned by any existing method. In such cases a full scale model of the surface must first be built, and sheets of Mylar plastic are then positioned upon the tool, and the edge of the part and features are marked on the sheets. The resulting flat patterns are still not precise since the Mylar plastic does not deform in the manner of a fabric. Clearly, with the increased use of resin-matrix composite structures, a more accurate method for producing flat patterns is needed.